This section provides background information related to the present disclosure which is not necessarily prior art.
Lean burn engines provide improved fuel efficiency by operating with an excess of oxygen over the amount necessary for complete combustion of the fuel. Such engines are said to run “lean” or on a “lean mixture.” However, this increase in fuel economy is offset by undesired pollution emissions, specifically in the form of oxides of nitrogen (NOx).
One method used to reduce NOx emissions from lean burn internal combustion engines is known as selective catalytic reduction (SCR). SCR, when used, for example, to reduce NOx emissions from a diesel engine, involves injecting an atomized reagent into the exhaust stream of the engine in relation to one or more selected engine operational parameters, such as exhaust gas temperature, engine rpm or engine load as measured by engine fuel flow, turbo boost pressure or exhaust NOx mass flow. The reagent/exhaust gas mixture is passed through a reactor containing a catalyst, such as, for example, activated carbon, or metals, such as platinum, vanadium or tungsten, which are capable of reducing the NOx concentration in the presence of the reagent.
An aqueous urea solution is known to be an effective reagent in SCR systems for diesel engines. However, use of such an aqueous urea solution and other reagents may include disadvantages. Urea is highly corrosive and attacks mechanical components of the SCR system, such as the injectors used to inject the urea mixture into the exhaust gas stream. Urea also tends to solidify upon prolonged exposure to high temperatures, such as encountered in diesel exhaust systems. Solidified urea may accumulate in the narrow passageways and exit orifice openings typically found in injectors. Solidified urea may foul moving parts of the injector and clog any openings, rendering the injector unusable. Solidified urea may also cause backpressure and emission reduction issues with a system. This concern exists because the reagent creates a deposit instead of reducing the NOx.
Several current injector systems include mounting arrangements that position the injector a predetermined distance away from the exhaust pipe. Some injector mounting arrangements may be referred to as a “dog house” or “stand-off” style. This mounting arrangement may introduce re-circulating vortices and cold spots at or near the injector mounting site and the reagent exit orifice. During urea injection, the re-circulating vortices and reduced temperature in the mount area may lead to reagent deposition that may clog the mount area and protrude into the exhaust gas stream.
In addition, if the reagent mixture is not finely atomized, reagent deposits may form in the catalytic reactor, inhibiting the action of the catalyst and thereby reducing the SCR system effectiveness. High injection pressures are one way of minimizing the problem of insufficient atomization of the urea mixture. However, high injection pressures often result in over-penetration of the injector spray plume into the exhaust stream, causing the plume to impinge on the inner surface of the exhaust pipe opposite the injector. Over-penetration leads to inefficient use of the urea mixture and reduces the range over which the vehicle can operate with reduced NOx emissions. Only a finite amount of reagent can be carried on a vehicle, and what is carried should be used efficiently to maximize vehicle range and reduce the need for replenishing the reagent.
Further, reagents may be poor lubricants. This characteristic adversely affects moving parts within the injector and requires that special fits, clearances and tolerances be employed between relatively moving parts within an injector. Some reagents have a high propensity for leakage. This characteristic adversely affects mating surfaces requiring enhanced sealing resources in many locations.
It may be advantageous to provide methods and apparatus for injecting a reagent into the exhaust stream of a lean burn engine to minimize reagent deposition and to prolong the life of the injector components.
The methods and apparatus of the present disclosure provide the foregoing and other advantages.